On various spinning and weaving machines it is known to use devices for sensing the flow of yarn to be woven (yarn feelers), many of these devices being piezoelectric devices.
In constructing any piezoelectric yarn feeler, the transducer element for sensing the movement of said yarn is very important.
In this respect, this transducer element generates the electrical signals produced by the movement of the yarn over said transducer, and also generates the electrical disturbance signals induced by the environment. Each transducer must therefore be constructed with the highest possible ratio or signals produced by the yarn flow to signals produced by "environmental disturbance", the term "environmental disturbance" indicating any electrical signal generated by the piezoelectric transducer which is other than that produced by the movement of the yarn.
The choice of the shape and component materials of a transducer of this kind is therefore the determining factor in improving the signal/disturbance ratio for each application.
The practical embodiments used up to the present time for transducers for yarn feelers of spinning and weaving machines, of which FIG. 1 of the accompanying drawing is a fairly common example, are characterised in that the vibrations produced by the yarn f on a ceramic yarn guide 1 of the feeler are transmitted by a metal foil 2 to the ceramic piezoelectric element 3, the yarn guide 1 and ceramic piezoelectric element 3 being rigidly cemented to the foil 2.
The coefficient of transmission of the vibration from the yarn guide 1 to the ceramic element 3 therefore depends on the mechanical coupling between the component parts of the transducer and on the type of adhesive used, and gives rise to large losses with regard to the signal produced by the flow of the yarn.